Piezoelectric transformers utilizing the piezoelectric effect are known in the art. A piezoelectric transformer can be defined as a passive electrical energy transfer device or transducer employing the piezoelectric properties of a material to achieve the transformation of voltage or current or impedance. Piezoelectric transformers have recently replaced wound-type electromagnetic transformers for generating high voltage in the power circuits for certain electronic applications. These piezoelectric transformers offer numerous advantages over ordinary electromagnetic transformers including a compact and slim shape, rugged construction, and high efficiency and reliability in a comparably smaller package. Piezoelectric transformers are particularly well suited for high voltage applications.
Piezoelectric ceramic transformers are now finding applications in a variety of applications including photocopiers, backlights for liquid crystal displays, flat panel displays, power converters, CRT displays, and the like. FIG. 1 shows the construction of a Rosen-type piezoelectric transformer, a representative example of a piezoelectric transformer of the prior art.
FIG. 1 shows a standard multilayer ceramic piezoelectric transformer 100 formed from a co-fired package of multilayer ceramic green tape interspersed with layers of a conductive metallization electrode pattern 102. Although the transformer package is made from layers of a green ceramic tape, upon firing, the transformer package sinters into a single ceramic structure. Consequently, all FIGs. will show the fired package structure and the individual layers of green tape will not be shown. An input voltage (V.sub.IN) is applied across the electrode layers 104 creating a potential difference across the driving region 106. The driving region 106 is polarized in the direction of its thickness, as indicated by the arrows between the electrode layers 104 shown in FIG. 1.
Polarization is a process wherein a very substantial direct current (DC) voltage, in the range of 4 Kv/mm, is applied to the ceramic in order to give the material its piezoelectric properties. Similarly, a portion of the piezoelectric transformer 100 indicated by reference number 108 is a power generating or driven section. An output voltage (V.sub.OUT) is formed on an end face 110 of the piezoelectric transformer 100 corresponding to the power generating section 108. Note that the end face 110 is metallized with a conductive coating, as is the top surface 112 in the driving section 106 of the piezoelectric transformer 100. Electrode layers 104, internal to the multilayer package, are also made from a conductive coating material The power generating section 108 is polarized in the lengthwise direction, as indicated by an arrow in FIG. 1.
With reference to FIG. 1, the basic operation of a piezoelectric transformer can be understood. When a voltage is applied to the driving section 106 of the piezoelectric transformer, the resulting electric field causes a vertical vibration due to a change in the thickness of the driving section 106 of the transformer. This vertical vibration results in a horizontal vibration in the lengthwise direction (also known as the piezoelectric transverse effect 31 mode) along the entire length of the transformer 100. In the power generating section 108 (also referred to as the driven section), a voltage having the same frequency as that of the input voltage (V.sub.IN) is derived through the output electrode (V.sub.OUT) in accordance with a piezoelectric effect wherein a potential difference occurs in the polarizing direction due to the mechanical strain in the polarizing direction. At this time, if the driving frequency is set to be the same as the resonant frequency of the piezoelectric transformer, a very high output voltage can be obtained. Stated another way, by applying an alternating current through the driving section 106 of the piezoelectric material, its thickness characteristics will vary, which through coupling causes a change in length of the driven section 108, which results in a change in the electrical output caused by an electro-mechanical effect.
Many prior art piezotransformers, such as the one shown in FIG. 1, measure a feedback from the output voltage (V.sub.OUT) of the transformer. Typically, the feedback signal is sent through a feedback network, an oscillator, and an amplifier (see FIG. 1). Significant advantages, as will be discussed below, can be achieved when the feedback is measured from another location on the transformer, independent of load. Moreover, the feedback signal may more accurately track resonance when the feedback signal is isolated from the load.
A problem encountered in the manufacture of piezoelectric transformers for certain electronic applications involves assuring that the transformer is driven at its resonant frequency. The "Q" property ("Q" is defined as the center frequency divided by the bandwidth) of piezoelectric type transformers is extremely high compared to the "Q" properties of magnetic type transformers. "Q" values of several hundred or more are not unusual for piezoelectric type transformers. Since high "Q" values are associated with a narrow bandwidth, maintaining the resonant frequency for a piezoelectric type transformer (high "Q") becomes considerably more difficult than maintaining the resonant frequency for a magnetic type transformer (low "Q").
In most piezotransformer applications, the transformer must be operated at or very near its resonant frequency. In some applications, frequency errors of less than 0.5% can be detrimental and render the transformer inoperable. Thus, the ability to track frequency through a voltage feedback can be of great importance for many applications. The ability to keep a transformer "on frequency" may be necessary to assure the operation of the transformer.
Unfortunately, there are many ways in which a transformer's resonant frequency may change during operation of the device. The resonant frequency of the piezoelectric transformer may be affected by many factors such as the output load variation, drive level, mounting technique, and temperature.
A piezoelectric transformer with a voltage feedback offers a solution to this problem. It is possible to operate the transformer in closed-loop self oscillation circuit. In this case, the oscillation occurs at a frequency for which the phase at the circuit input is the same as the phase at the circuit output. Another possibility involves operating the transformer in a closed-loop oscillation circuit in which a voltage controlled oscillator (VCO) or other type of oscillator is forced to operate at a frequency determined by the phase of the feedback signal with respect to the circuit input. In both cases, the phase shift through the piezotransformer must be known so that standard phase shift networks can be used to close the feedback loop and maintain frequency stability and tracking characteristics.
Piezoelectric transformers with voltage feedbacks are known in the art. Feedback tabs are commonly described in the prior art as being necessary to provide a relatively low voltage signal in lieu of the high voltage output (V.sub.OUT) of the transformer. Moreover, feedback tabs are also necessary to provide a signal with a constant phase relationship with respect to the input drive signal (V.sub.IN) of the transformer. This allows the resonant frequency of the transformer to be maintained. However, prior art voltage feedback designs have some fundamental drawbacks, as will be described below, which render them inapplicable for many piezotransformer applications.
In prior art piezotransformers, a percentage of the output voltage or current is oftentimes used for the feedback signal Because the phase of the output signal, with respect to the input drive signal, can change as the load varies, such an approach may only be employed for a constant load.
Also, in prior art piezotransformers where feedback tabs are used, the feedback signal is not truly independent of the load. This is due to the fact that the feedback signal reflects some of the phase changes that occur as the load changes.
Still another drawback with prior art piezotransformers is that the voltage feedback tab is physically located in an area of the transformer which couples energy from the input drive signal. In other words, the prior art feedback tabs are located in the driving section of the transformer. This has the adverse effect of allowing higher order harmonics to be fed back to the input section making it difficult to track resonance.
Additionally, another drawback with the prior art is that the feedback tab is physically located in an area of the transformer which supports spurious vibration modes. Moreover many prior art transformers have physical dimensions which do not suppress spurious vibration modes.
If a piezotransformer having a feedback voltage could be made independent from the output voltage, such that the phase of the feedback voltage with respect to the drive voltage was constant at resonance, then the resonant frequency for varying loads could be maintained.
A piezoelectric transformer with a voltage feedback section (voltage feedback tab) which can be used to keep the transformer "on resonance" and which is isolated by its strategic placement in the driven section of the transformer and which provides feedback control in the form of a feedback voltage which has a constant phase relationship for varying loads at resonance relative to the input voltage and which reduces harmonics at a frequency of interest and which provides a low-cost solution to a complicated frequency tracking problem by reducing the number of required electronic components would be considered an improvement in the art.